A semiconductor device 100 shown in FIG. 10 has a semiconductor element 101 such as IGBT mounted thereon. Since the semiconductor element 101 generates a large amount of heat, a substrate 102 is formed of a metal base substrate made of such as aluminum and copper having good thermal conductivity and electric conductivity or a substrate formed of a ceramic board. In the case of bonding the semiconductor element 101 to an upper surface of the substrate 102 by solder, a back surface of the semiconductor element 101 made of silicon is subjected to a metalization treatment using nickel alloy or the like in order to improve bonding strength with a solder layer 103. Further, a plated layer 104 is formed by nickel plating on a surface of the substrate 102 made of metal.
For instance, on the substrate 102 formed of a copper plate by press working, an outflow-preventing layer 105 is formed to ensure the coating thickness of the solder layer 103 to prevent solder from flowing out. The outflow-preventing layer 105 is made in such a way that a part of the substrate 102 which is not to be coated is masked and an exposed surface is coated by a spraying device. As a coating method of forming the outflow-preventing layer 105, a spraying method that forms a relatively stably thick coat (50 μm or less) is used. For the outflow-preventing layer 105, oxides such as aluminum, silicon, and titanium are used. The outflow-preventing layer 105 on the substrate 102 is formed to surround the semiconductor element 101 so that the outflow-preventing layer 105 hold back molten solder to prevent solder outflow.